The disclosure of Japanese Patent Application No. HEI 10-268674 filed on Sep. 22, 1998, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to an automatic transmission installed in a motor vehicle and, more particularly, to a hydraulic operator for a band brake within the automatic transmission, which band brake includes a flexible band strip and which is operated to perform a speed-change operation in shifting to a predetermined speed stage.
2. Description of the Related Art
In general, an automatic transmission is provided with a speed-change gear mechanism, a torque converter or the like. The speed-change gear mechanism is composed of planetary gearing. As shown in FIG. 1, a drum 1 is coupled to an element of the speed-change gear mechanism such that the drum 1 rotates integrally with the element. A band 2, which is a flexible band strip, is wound around the drum 1, and a hydraulic servo 3 is driven so that the band 2 tightens on the drum 1. In this manner, a speed-change operation is performed.
The band 2 is wound around the drum 1 with one end 2a being secured to a casing.
The hydraulic servo 3 has a cylinder casing 30 disposed in the casing of the automatic transmission and a piston 31 oil-tightly fitted within the cylinder casing 30. A return spring 34, which is interposed between one side of the piston 31 and the casing 30, urges the piston 31 leftwards in FIG. 1. Furthermore, an end plate 8 is axially and oil-tightly fitted to the end of the cylinder casing 30. The end plate 8 and the piston 31 define a brake engagement hydraulic chamber 32 therebetween. A rod 33, which is supported by the piston 31, projects from one side of the casing 30 and abuts a stop 2b on a second end 2c of the band 2. When a hydraulic pressure is supplied to the brake engagement hydraulic chamber 32, the rod 33 moves rightwards in FIG. 1, so that the band 2 tightens on the drum 1.
In the aforementioned automatic transmission M1, the drum 1 rotates clockwise in a de-energizing direction A (in the direction of counter-lock) at a certain speed stage, whereas the drum 1 rotates counterclockwise, in a self-energizing direction B (in the direction of self-lock) at another speed stage. The aforementioned hydraulic servo 3 is driven so that the band 2 acts either on the de-energizing side or on the self-energizing side to stop rotation of the drum 1. In this manner, a speed-change operation to a predetermined speed stage is performed.
However, in the automatic transmission employing the band 2, the state of engagement between the band 2 and the drum 1 changes discontinuously due to slippage until the drum 1 is finally stopped. As a result, there is a problem of oscillation (hereinafter referred to as xe2x80x9cjudderxe2x80x9d) with a frequency of about 30 Hz as well as fluctuation of the output torque. The problem associated with generation of judder is especially serious on the de-energizing side. The band 2 does not uniformly engage the drum 1 in tightening on the drum 1 and thus causes a change in frictional characteristic. Due to the aforementioned de-energizing effect, the rod 33 stops and starts when driven by the hydraulic servo 3. Consequently, fluctuation of output torque occurs. This is considered to be the cause of the aforementioned problem regarding generation of xe2x80x9cjudder.xe2x80x9d
It is thus an object of the present invention to provide an automatic transmission that prevents generation of xe2x80x9cjudderxe2x80x9d as well as fluctuation in output torque deriving from operation of a band brake.
According to the present invention, there is provided an automatic transmission with a band brake including a rotational body (xe2x80x9cdrumxe2x80x9d) rotating integrally with an element of a speed-change gear mechanism, a flexible band strip wound around the rotational body, and a hydraulic servo for operating the flexible band strip. In this automatic transmission, the hydraulic servo is driven such that the flexible band strip tightens on the rotational body in a speed-change operation. The hydraulic servo has a cylinder casing, a piston reciprocally mounted within the cylinder casing, a rod disposed to engage with the flexible band strip, and an elastic member disposed between the piston and the rod, whereby a hydraulic force is transmitted from the piston to the rod through the elastic member to tighten the flexible band strip on the rotational body. The elastic member is designed to absorb forces on the rod caused when a torque is being transmitted by the flexible band strip.
At a given speed-change stage where an element of the speed-change gear mechanism rotates integrally with the rotational body, the hydraulic servo is driven to transmit a hydraulic force from the piston through the elastic member to extend the rod. Thereby, the rod engages the flexible band strip, and the flexible band strip tightens on the rotational body. As a result, the element of the speed-change gear mechanism and the rotational body to which it is fixed are stopped from rotating, and the speed-change operation is performed. Even if the state of engagement of the flexible band strip on the rotational body is uneven, e.g. due to slippage, prior to stoppage of the rotational body, the elastic member absorbs displacements (oscillation) of the rod.
According to the present invention, when the flexible band strip tightens on the rotational body, the elastic member absorbs oscillation of the rod. Thus, the flexible band strip tightens on the rotational body with a substantially constant tightening force, whereby generation of judder as well as fluctuation of output torque is prevented.
In the present invention the rod is supported by the piston for axial movement relative thereto, and the elastic member is a disc spring interposed between the piston and the rod. So constructed, the disc spring serves to prevent oscillation of the rod. Therefore, it is possible to effectively absorb any oscillation of the rod, utilizing an elastic member having a large modules of elasticity, without increasing the size of the hydraulic servo.
Further, in embodiments wherein the elastic member is composed of at least two springs which have different moduluses of elasticity and are connected in series, the two springs serve to absorb oscillation of the flexible band strip both on the self-energizing side and on the de-energizing side. Accordingly, it is possible to effectively prevent generation of judder both in self-energizing and de-energizing.
The present invention also provides a supporting plate disposed between the piston and the rod for movement in the same direction as the piston, a first spring disposed between the supporting plate and the rod, and a second spring disposed between the supporting plate and the cylinder casing coaxial with the first spring, and the elastic member is disposed between the supporting plate and the piston. In this construction, unless the hydraulic force has reached a predetermined value, the second piston returns the first spring. Conversely, if the hydraulic force has become equal to or exceeded the predetermined value, the first spring serves to absorb impact.